Production of uranium tetrafluoride



Nov. 26, 1968 J. A. RODE PRODUCTION OF URANIUM TETRAFLUORIDE Filed Nov.

INVENTOR JAMES A. RODE ATTORN Y Pile. g

United States Patent York Filed Nov. 14, 1966, Ser. No. 594,201 11 Claims. (Cl. 23-353) ABSTRACT OF THE DISCLOSURE High bulk density UK, is produced by converting UF in a fluidized bed reactor in which U-F particles are suspended by gaseous mixtures of UF tetrachloroethylene or an equivalent organic fluorine acceptor containing 1 to 3 carbon atoms, and a diluent, nitrogen. The temperature within the fluidized bed is in the range of 500 F. to 600 F., and the amount of tetrachloroethylene introduced into the fluidized bed is suflicient to provide a stoichiometric excess of about 9% to 12% for the conversion of UF to UF The excess U1 particles generated in the fluidized bed are removed therefrom.

This invention relates to high bulk density uranium tetrafluoride and, more particularly, to a new process for producing the same.

In the production of ceramic grade U0 from UF the hexafluoride is initially reduced to the tetravalent form, and the latter is then converted by pyrohydrolysis to uranium dioxide. For economic production, the intermediate product, uranium tetrafluoride, must be substantially free of other forms of fluoride contaminants such as UF UgFq, and U 1 and has a relatively high bulk density. A preferred type of prior processes for the preparation of uranium tetrafluoride from UF involves a vaporphase reaction using hydrogen, carbon tetrachloride, ethylene dichloride, trichloroethylene or chloroform as a reductant for the conversion of UF to its tetravalent form. To avoid the formation of undesirable UP and other forms of uranium fluorides in the process, the reaction must be closely controlled. The UR, thus produced is an extremely fine powder of low bulk density.

The low bulk density uranium tetrafiuoride powder is diflicult to handle and the uranium dioxide produced therefrom has similar low bulk density physical characteristics which are undesirable for the preparation of ceramic grade U0 pellets used in the nuclear fuel elements. Attempts to increase the bulk density in the vapor-phase type process have not been very successful. The bulk density of UF powder produced according to some of the modified vapor-phase reactions is still substantially below the desired value.

In the process of this invention, a high bulk density uranium tetrafluoride is produced from uranium hexafluoride which is substantially devoid of other forms of uranium fluoride contaminants. Broadly stated, the process involves the introduction of gaseous UP and a gaseous organic fluorine acceptor containing 1 to 3 carbon 'atoms into a fluidized bed of UF solid particles. The concentration of the gaseous UP and the organic fluorine acceptor and the temperature in the reaction zone are regulated to promote a surface-reaction on the solid UF particles. A substantially constant volume of the fluidized bed is maintained by continuously withdrawing the UF particles generated therein. Preferably, the reaction is carried out at a temperature in the range between about 500 F. and about 600 F.

Suitable organic fluorine acceptors are those containing 1 to 3 carbon atoms and are in the form of a vapor at the reaction temperature. Preferably, they are halogenated hydrocarbons containing an unsaturated chain. I found compounds of the following structural formula to be particularly desirable:

wherein X is F or Cl. Using either C1\ 01 0:0 C1 C1 F 01 C1 C1 for the reduction of UF the by-product is the valuable Freon.

In the process of this invention, the reaction of UF and the organic fluorine acceptor is predominantly a coating type or a surface induced reaction. The UP, from the reduction operation is produced in the form of a coating on the UR; seed in the fluidized bed. Attrition of the large particles or fines generated by the minor vaporphase operation produces additional seed or surface area for the reaction to replace the surface area that is lost by particle growth and by the particles removed from the bed.

The reaction rate is a function of the temperature and concentration of the reactants. 1 have found that the rate of the vapor-phase reaction of UF and the organic fluorine acceptor increases more rapidly than the surface induced reaction as the temperature or concentration increases. The reaction, therefore, is advantageously carried out at a temperature and concentration where the vapor-phase reaction is at a minimum while the surface reaction is at its maximum. It is noted that a minor amount of vapor-phase reaction is tolerable because it generates additional seeds or reaction surface upon which the new green salt is deposited causing particle growth. Hence, the temperature and concentration may be varied individually or jointly to control the reaction so that it will provide a stable surface area in the fluidized bed. It is found that the reaction is best controlled by the variation of concentration. This is because variation of the reaction rate caused by the fluctuation of the concentration is more moderate compared to that caused by the change of temperature.

The critical limitation in this process is the minimum residence time of the organic fluorine acceptor which is dependent on the temperature and concentration. The

3 4 residence time in the fluidized bed is defined by the folhence, total surface area. For a seed bed using the +100 lowing equation: mesh particles, 26 to about 35 pounds of UF may be used. The amount of C 01 used is more than the stoichiometric amount required for the conversion of UF 5 introduced into the reactor. The percentage of excess may vary depending on the reaction variables. A molar excess at a residence time below a certain value leads to excess of more than 100 Pew?nt may be used as determined by fine production and formation of undesirable UF the gas from the reactor- The residence time is varied for each organic fluorine XA L 1 acceptor. It was found that the minimum residence time is 1.0 second for the reaction of UF and Cl C=CCl at a temperature in the range of about 500 F. to about volume of the fluidized bed Resldence tune: flow rate of the gas I found that in carrying out the process of this invention This example is used to illustrate the computation of residence time based on the minimum conditions for UP to UF conversion using C Cl Further to illustrate this invention specific examples are UF fiow 34 lb./hr. (Does not enter into calculation bedescribed hereinbelow with reference to the accompanying cause UF is a solid.) drawings wherein N flow: S.c.f.h. FIG. 1 is a photograph of UF particles produced in Nozzle 30 a typical run in accordance with the invention, magnified Annulus 30 800 times, and Fluidizing gas 72 FIG. 2 is a fluidized reactor partly in section used for Purges 10 V the production of the UH; particles.

In these examples, tetrachloroethylene is used as the Total N 142 138.5 c.f.h.

Perclene flow 1.3 g.p.h.:

1.3 g.p.h. 13.55 lb./gal. 359 cu.ft./mole 166 1b./lb. mole Perolene flow c.f.h. at reactor temp. and press:

960 R. 15 p.s.i.

X4920 R X :=37.2 c.f.h.

175.7 0 f.h. 2 o o Total flow (N perclene) 1o8.5+o7.2 17o.7 e.f.h. or 3600 Sea/hr.

= 0.04885 cu.ft./sec.

2 2 Cross-sectional area. of 4-. bed= f= =l2.57 sq. in.

Bed height above n0zz1e=21.5 inches Bed volume=H A=21.5 12.57=27O cu. in.

270 cu. in. m.-O.1564 cu. ft.

vol. bed 0.1564 cu. ft. Resldence time m bed flow 004885 cu. ft./seo.

= 3.21 sec.

organic fluorine acceptor. In carrying out the process, the EXAMPLE 2 fluidized reactor 10 is initially loaded with UF particles about +100 mesh in size. The reactor 10 is substantially in the form of a cylinder having an interior diameter of This example is used to illustrate a typical run using the following operational conditions:

b t 4 in h s, The top portion of the reactor 10 i d Weight (Hm) equipped i a plurality of filters 11 having gas ou le s Tate (ii pipes 12 in communication with the interior of the filters. Run t 18.0 One or more of the filters may be used at a time while a ReaChOIl p- 500-550 flow of gas is passed in reverse through the other filters to Gas composition (P remove the dust collected thereby. 0 The fluidizing gas, nitrogen, and tetrachloroethylene ar 2 75 introduced into the lower chamber 12 0f the flui C3014 (in) 17O reactor through gas inlet 13. The IOWEI' chamber 12 is C2c14 (out) separated by a diffusion disk 14 which has a central hole During the fi t six hours f the run the particle Size 15 connected to the product line 16 from which the excess increases to a level of 97% +200 mesh Aft 6 hours, 4 P213161es are q e A mlxthhe of prehehthd fines production began leveling off at approximately 60% g and gaseous e 1S Introduce? Into the fluldlzed bed fine apparently because the bed volume and/or surface through 1102216 17 Connected t0 P P Thetfeed nozzle area had been reduced through minor losses of fines from 17 y he a dual gas nozzle cohslstmg an Inner nozzle the bed and/ or growth of particles. The screen analysis for UF diluted with nitrogen and an annular nozzle for f the product showed less than 2% f 00 mesh and nitr0g n SHITOUHdiHg the h type of nozzle about 30% 200 mesh. The particles have a particle denhas th advantage of y g the mlXlIlg 0f the tetrasity of about 6.7 g./cc. which is essentially the theoretical chloroethylene and UP thereby eliminating reaction at value f r the density of UF A photograph of the parthe nozzle. ticles with 800x magnification is shown in FIG. 1. The

The amount of seed used depends on the particle size, bulk density of the UH; particles is about 3.5 g./cc.

5 EXAMPLE 3 This example is used to illustrate the growth of UF particles during a typical run using the following operational conditions. The growth of the particles was stabilized after about 14 hours:

Seed weight, 45 lbs. mesh -100200 UF feed rate lbs./hr 12 Run time hours 24 Reaction temp. F.):

Immediately above the gas distributor 530-560 Balance of reactor 560-600 Gas Flow, c.f.h. Percent in Percent out UF 12. 24 10. 7 N2 82 71. 5 80 (2014 (in) 20.51 17.8 C2014 (out) 8. 27 8.

I clann 1. A process for the conversion of UP to -UF. which comprises introducing gaseous UF and a gaseous organic fluorine acceptor containing 1 to 3 carbon atoms into a fluidized reaction zone having fluidized UF solid particles, regulating the concentrations of the gaseous UF and the organic fluorine acceptor and the temperature in said reaction zone to promote a surface-reaction on said solid UF particles, and removing the excess UF particles in the reaction zone generated by the reaction of UF and said organic fluorine acceptor.

2. A process according to claim 1 wherein the reaction temperature is maintained in the range between about 500 F. and about 600 F.

3. A process according to claim 2 wherein the organic fluorine acceptor has the structural formula:

wherein X is C1 or F, and nitrogen is used as a fluidizing gas.

4. A process according to claim 3 wherein the organic fluorine acceptor has the structural formula:

and the amount of said organic fluorine acceptor in the reaction zone is maintained in a stoichiometric excess required for the UB to UB conversion reaction.

5. A process according to claim 4 wherein the minimum residence time for the organic fluorine acceptor in the reaction zone is about 1.0 second.

6. A process for the conversion of UP to UF which comprises introducing into a fluidized bed of UP; particles gaseous UF and a gaseous organic fluorine acceptor conwherein X is C1 or F, and nitrogen is used as a fluidizing gas.

8. A process according to claim 6 wherein the organic fluorine acceptor has the structural formula:

9. A process according to claim 8 wherein the organic fluorine acceptor has a minimum residence time in the fluidized bed of about 1.0 second.

10. A process for producing high bulk density which comprises introducing into a fluidized bed of UF particles a gaseous mixture of nitrogen and UP and a gaseous mixture of nitrogen and tetrachloroethylene, maintaining the temperature within the fluidized bed in the range of 500 F. to 600 F., regulating the amount of tetrachloroethylene introduced to said fluidized bed to provide a stoichiometric excess about 9% to 12%, and having a minimum residence time of 1.0 second, and withdrawing the excess UF particles generated by the conversion of UP to UF from the fluidized bed.

11. A high bulk density UF produced according to claim 10 of which less than 2% of the particles is finer than -200 mesh and has a bulk density about 3.5 gm./ cc.

I. E. Baker, H. V. Klaus, R. A. Schmidt, S. H. Smiley: A Pilot Plant For the Reduction of UP to U1 With Trichloroethylene, Report K1271, 1956, Oak Ridge Gaseous Diffusion Plant.

CARL D. QUARFORTH, Primary Examiner.

M. J. MCGREAL, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,413,099 November 26, 1968 James A. Rode It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3, line 35, "Total flow (N perclene) should read Total flow (N perclene) Column 5, after line 12 insert Recycle 33% -lOO 200 mesh. Column 6, line 1, after "density" insert UF Signed and sealed this 10th day of March 1970.

(SEAL) Attest:

WILLIAM E. SCHUYLER, JR.

Edward M. Fletcher, Jr.

Commissioner of Patents Attesting Officer 

